Filter cake removal composition for drilling fluids and method of use thereof

ABSTRACT

Disclosed is a filter cake removal composition, and method of use thereof, for use in a wellbore for controlled removal of a filter cake present in a target production zone. The filter cake removal composition includes hydrochloric acid and an organic acid. The filter cake removal composition, when a mixture of the hydrochloric acid and the organic acid is applied to the filter cake in the target production zone, is operable to dissolve the filter cake in the target production zone over an extended reaction time. The mixture includes the hydrochloric acid present in an amount of between about 0.1% and 5% by weight and the organic acid present in an amount of between about 0.1% and 10% by weight.

RELATED APPLICATION

This application is related to, and claims priority to, U.S. ProvisionalPatent Application Nos. 61/567,983 and 61/567,993, both provisionalapplications filed on Dec. 7, 2011, the disclosures of which areincorporated herein by reference in their entirety.

BACKGROUND

1. Field of the Invention

Embodiments of the invention generally relate to methods andcompositions for removing a completion fluid filter cake in a wellbore,and more particularly, to methods and compositions for dissolving orremoving filter cake material generated by a manganese-tetraoxide-baseddrilling fluid in a wellbore for optimizing production from asurrounding hydrocarbon bearing formation.

2. Description of the Related Art

Horizontally/multilaterally-drilled wells have been used to enhance bothhydrocarbon recovery and total well productivity from many types ofreservoirs. Drilling, workover, and production operations may result innear-wellbore formation damage that in most cases cannot be prevented(e.g., pore plugging by calcium carbonate particles from drilling fluid,drilled solid particles, or particles from the formation).

During well operations, drilling fluids can be lost into the surroundingformation. To prevent this, the drilling fluid is frequently modifiedsuch that a small amount of the fluid and solids contained therein forma coating on a wellbore surface (i.e., the formation of a “filtercake”). After the completion of drilling operations, the coating orfilter cake is typically removed, and production from the formation canproceed. The process used to remove the filter cake can also be used toremove other types of damage or debris from the wellbore prior tobeginning hydrocarbon production.

To facilitate the drilling of horizontal/multilateral wells, weightingmaterials have been introduced into the drilling fluid to increase thedensity of the drilling fluid for balancing the hydrostatic pressure andfor maintaining stability within the wellbore to minimize formationdamage and corrosion in the wellbore. Several weighting materials (e.g.,bentonite, barite, calcium carbonate (CaCO₃) ilmenite, and hematite)have been used in drilling fluids, each of which has several associatedlimitations. For example, bentonite and barite are not soluble inhydrochloric acid (HCl), and therefore they may cause formation damagein the wellbore. The specific gravity of CaCO₃ (e.g., 2.71) limits itsapplication when a high density drilling fluid is needed to drill deepwells. Due to the partial solubility of barite in concentrated formatebrines and the conventional practice not to acidize the wellbore priorto completion of the well, CaCO₃ and barite have been excluded asoptions to increase density of the drilling fluid in many applications.

Manganese tetraoxide (Mn₃O₄) is a high density, acid-soluble weightingmaterial useful in drilling fluids for drilling high temperature/highpressure (HT/HP) wells. Mn₃O₄ is spherical in shape and has an averagepartial size of 1-5 microns and a specific gravity of 4.8, which make itappropriate for drilling deep wells. For example, Mn₃O₄ has beenintroduced into potassium formate drilling fluids to overcome the maindrawback of potassium formate, which is the production of a brine ofdensity 1.7 g/cm³ (106 lb/ft³). Mn₃O₄ has also been introduced as aweighting material into oil-based drilling fluids due to its ability tolower the plastic viscosity of the oil-based drilling fluid. Awater-based drilling fluid weighted with Mn₃O₄ and a small amount ofCaCO₃ has also been formulated for use in a wellbore. CaCO₃ has beenadded to the water-based drilling fluid to control filtration propertiesof the drilling fluid. The need in the industry for a drilling fluidwith high rheological properties has been achieved using Mn₃O₄particles.

Mn₃O₄ particles, however, also present many disadvantages as a weightingmaterial in oil-based or water-based drilling fluids. For example, Mn₃O₄particles aggregate up to 20 microns in aqueous and oil-based fluids.Accumulation of these aggregates in the critical near wellbore area canresult in stuck pipe and mud cake problems during drilling operations.Dust problems associated with the accumulation of these aggregates havealso caused formation damage in the wellbore. Additionally, starch maybe present in the filter cake covered Mn₃O₄ particles, which cancontribute to additional particle agglomeration. Thus, addressing theremoval of a filter cake formed by a drilling fluid weighted with Mn₃O₄particles is essential to ensure the effectiveness of drilling andcleaning operations in the wellbore.

Several cleaning compositions have been developed to remove the filtercake generated by a manganese-tetraoxide-based drilling fluid from thewellbore and to minimize formation damage in the wellbore using liveacids, gelled acids, strongly buffered organic acids, chelating agents,oxidizing agents, enzymes, in-situ generated organic acids,microemulsions, or combinations of these chemicals. Because Mn₃O₄ is astrong oxidizing agent having an active phase (i.e., a tetragonalsymmetry, non-stoichiometry behanvior) locally composed of an octahedralMn₂O₃ phase and a tetrahedral MnO phase, it experiences complexinteractions with most cleaning fluids, including the aforementionedchemicals. For example, organic acids and chelating agents will notindependently dissolve Mn₃O₄-based filter cakes. Ethylene diaminetetracetic acid (EDTA) at high pH (e.g., a pH of 12) and acetic,propionic, butyric, and gluconic acids at low pH (e.g., a pH of 3-5)exhibit very low solubility. Glutamic, citric, oxalic, and tartaricacids produce white precipitation when reacted with Mn₃O₄ particles.Similarly, diethylene triaminen pentaacetic acid (DTPA) precipitatesmanganese silicate if used to dissolve Mn₃O₄-based filter cake in asandstone formation.

Citric acid in an amount of about 10% by weight has been used as acleaning fluid for effectively removing Mn₃O₄-based filter cakes.However, when reacted with Mn₃O₄, citric acid has been known todissociate insoluble manganese citrate causing formation damage in thewellbore, and therefore is not a suitable composition to effectivelydissolve or remove the Mn₃O₄-based filter cake from the wellbore whilepreventing formation damage.

Therefore, what is needed is a filter cake removal composition whichdissolves, and more preferably removes, a filter cake generated by amanganese-tetraoxide-based drilling fluid without causing formationdamage in the wellbore.

SUMMARY

Embodiments of the invention are directed to compositions, and methodsof use thereof, for dissolving or removing filter cake materialgenerated by a manganese-tetraoxide-based drilling fluid in a wellborefor optimizing production from a surrounding hydrocarbon bearingformation. An embodiment of the invention includes a filter cake removalcomposition including HCl having a concentration lower than 5% by weightfor partially dissolving the Mn₃O₄-based filter cake. According toanother embodiment of the invention, a two-stage filter cake removalcomposition is provided which includes an enzyme that is first appliedto the filter cake, followed by the application of an organic acid tothe resulting filter cake. In accordance with another embodiment of theinvention, there is provided a single-stage filter cake removalcomposition including a mixture of HCl and an organic acid fordissolving a portion of the filter cake in the wellbore, which will bedescribed in more detail below. According to another embodiment of theinvention, there is provided a two-stage filter cake removal compositionthat has been demonstrated to remove filter cake material generated by amanganese-tetraoxide-based drilling fluid in a wellbore using theapplication of an amylase enzyme in a first stage, followed by theapplication of a mixture of HCl and an organic acid in a second stage,which will also be described in further detail below. Methods of use ofthe latter two filter cake removal compositions will also be describedin further detail below.

In particular, there is provided a filter cake removal composition, inaccordance with an embodiment of the invention, for use in a wellborefor controlled removal of a filter cake present in a target productionzone. The filter cake removal composition includes hydrochloric acid andan organic acid. The filter cake removal composition, when a mixture ofthe hydrochloric acid and the organic acid is applied to the filter cakein the target production zone, is operable to dissolve the filter cakein the target production zone over an extended reaction time. Themixture includes the hydrochloric acid present in an amount of betweenabout 0.1% and 5% by weight and the organic acid present in an amount ofbetween about 0.1% and 10% by weight.

In accordance with another embodiment of the invention, there isprovided a method for the controlled removal of a filter cake from atarget production zone of a wellbore using a filter cake removalcomposition. The method includes delivering the filter cake removalcomposition to the target production zone. The step of delivering thefilter cake removal composition includes applying the filter cakeremoval composition to the filter cake for an extended reaction timeduring which extended time the filter cake removal composition acts todissolve the filter cake and after which extended time the filter cakeremoval composition acts to control fluid loss from the wellbore intothe target production zone. The step of delivering the filter cakeremoval composition further includes applying a mixture of hydrochloricacid and an organic acid to the filter cake in the target productionzone. The mixture includes the hydrochloric acid present in an amount ofbetween about 0.1% and 5% by weight and the organic acid present in anamount of between about 0.1% and 10% by weight.

BRIEF DESCRIPTION OF DRAWINGS

So that the manner in which the features and advantages of theinvention, as well as others which will become apparent, may beunderstood in more detail, a more particular description of theinvention briefly summarized above may be had by reference to theembodiments thereof which are illustrated in the appended drawings,which form a part of this specification. It is to be noted, however,that the drawings illustrate only various embodiments of the inventionand are therefore not to be considered limiting of the invention's scopeas it may include other effective embodiments as well.

FIG. 1 shows a filter cake before and after an application of a filtercake removal composition, including 10% by weight HCl, in accordancewith an embodiment of the invention.

FIG. 2 shows a gas chromatography/mass spectrometry graph illustratingthe relationship between the reaction of HCl with Mn₃O₄ particles andthe production of chlorine gas, in accordance with an embodiment of theinvention.

FIG. 3 is a graph showing the effect of Mn₃O₄-high soluble organic acidson the dissolution of manganese ions, in accordance with an embodimentof the invention.

FIG. 4 shows a filter cake before and after an application of atwo-stage filter cake removal composition, in accordance with anembodiment of the invention

FIG. 5 shows a graph illustrating the efficacy of a mixture of HCl andlactic acid for the dissolution of manganese ions from a Mn₃O₄-basedfilter cake, in accordance with an embodiment of the invention.

FIG. 6 shows a graph illustrating the solubility of a Mn₃O₄-based filtercake and acid concentration as a function of time using a two-stagefilter cake removal composition, in accordance with an embodiment of theinvention.

FIG. 7 shows the efficacy of a single-stage filter cake removalcomposition, in accordance with an embodiment of the invention, forremoving Mn₃O₄ particles from a filter cake.

DETAILED DESCRIPTION

Although the following detailed description contains many specificdetails for purposes of illustration, it is understood that one ofordinary skill in the relevant art will appreciate that many examples,variations, and alterations to the following details are within thescope and spirit of the invention. Accordingly, the exemplaryembodiments of the invention described herein are set forth without anyloss of generality, and without imposing limitations, relating to theclaimed invention. Like numbers refer to like elements throughout. Primenotation, if used, indicates similar elements in alternativeembodiments.

As used herein, the term “drilling fluid” shall be used to collectivelyrefer to a completion fluid or a drilling fluid. As understood in theart, “drilling fluid” shall be used to describe a fluid used to aid inthe drilling of a borehole for a well (e.g., a horizontal/multilateralwell). The drilling fluid may include a water-based mud (e.g., adispersed or non-dispersed water-based mud), a non-aqueous mud (e.g., anoil-based mud), and a gaseous drilling fluid.

Experimentation of conventional cleaning fluids demonstrates that Mn₃O₄deposits dissolve in concentrated (e.g., ˜12N) or dilute (e.g., ˜3N) HClsolutions. The reaction of HCl with Mn₃O₄ depends on the stoichiometryof the reactants as shown, for example, in Equations (1)-(3):

Mn₃O₄+8HCl

2MnCl₂+MnCl₄+4H₂O  (1)

3Mn₃O₄+12HCl

6MnCl₂+MnO₂+6H₂O  (2)

Mn₃O₄+6HCl

MnCl₂+MnCl₄+3H₂O  (3)

Embodiments of the invention demonstrate that HCl in an amount of about10% by weight may dissolve about 78% by weight of a Mn₃O₄-based filtercake at about 250° F. after 28 hours of soaking the filter cakematerial. However, corrosive/poisonous chlorine gas is detected duringthe reaction of between about 5 and 15% by weight of HCl with Mn₃O₄, asgiven, for example, by Equation (4):

Mn₃O₄+8HCl

3MnCl₂+Cl₂+4H₂O  (4)

Certain embodiments of the invention demonstrate that, at about 190° F.,an amount of between about 1 and 4% by weight HCl may dissolve most ofthe Mn₃O₄ particles in the filter cake (e.g., up to about 70% by weightsolubility), without the release of chlorine gas. The reaction of HCl inan amount of between about 1 and 4% by weight and Mn₃O₄ particles atabout 190° F. may be represented by equation (5):

Mn₃O₄+4HCl

2MnCl₂+MnO₂+2H₂O  (5)

Therefore, HCl at a concentration of less than about 5% by weight willpartially dissolve a Mn₃O₄-based filter cake.

Embodiments of the invention further demonstrate that about 4% by weightof an organic acid, such as lactic or formic acid, as non-limitingexamples, may dissolve Mn₃O₄ particles up to about 76% by weightsolubility at about 190° F. However, it has been observed that the Mn₃O₄particles of the filter cake were covered with polymeric material (e.g.,starch), which significantly reduces the solubility of the filter cakein the organic acids. Therefore, there is a need to remove the polymericmaterial on the Mn₃O₄ particles of the filter cake to more effectivelydissolve or remove the filter cake material in the wellbore. Certainembodiments of the invention provide for the application of an enzyme(i.e., an enzyme that catalyzes the breakdown of starch into sugars) toremove the polymeric material present on the Mn₃O₄ particles of thefilter cake.

Certain embodiments of the invention provide for a filter cake removalcomposition, and method of use thereof, for use in a wellbore for thecontrolled removal of a filter cake present in a target production zone.Generally, the filter cake removal composition includes low and safeconcentrations of HCl combined with an organic acid. The filter cakeremoval composition, when a mixture of the HCl and the organic acid isdelivered or applied to the filter cake in the target production zone,may be operable to dissolve the filter cake in the target productionzone over an extended reaction time, and subsequently may be operable tocontrol fluid loss from the wellbore into the target production zone.For example, the filter cake removal composition can be delivered orapplied to the filter cake in a single-stage treatment, whereby thefilter cake is allowed to soak in the cleaning fluid for up to about 24hours. According to an embodiment of the invention, the hydrochloricacid is present in an amount of between about 0.1% and 5% by weight andthe organic acid in an amount of between about 0.1% and 10% by weight.

In accordance with another embodiment of the invention, the hydrochloricacid is preferably present in an amount of about 1% by weight. Theorganic acid may be selected from the group consisting of formic acid,lactic acid, and malonic acid. In accordance with another embodiment ofthe invention, the organic acid includes lactic acid and is preferablypresent in an amount of about 4% by weight.

According to one embodiment of the invention, about 1% by weight HClcombined with about 4% by weight lactic acid may dissolve, in a singlestage treatment of the filter cake, about 85% by weight of theMn₃O₄-based filter cake after about 18-22 hours of soaking at atemperature of about 250° F. According to other embodiments, the singlestage treatment of the Mn₃O₄-based filter cake is performed in areservoir having temperatures higher than 250° F., while in otherembodiments, the reservoir has a temperature lower than 250° F.

Other embodiments of the invention provide a two-stage filter cakeremoval composition, and method of use thereof, for use in the wellborefor controlled removal of the filter cake present in the targetproduction zone. Generally, the two-stage filter cake removalcomposition includes an enzyme and a mixture of HCl and an organic acid.The two-stage filter cake removal composition, when the enzyme isdelivered or applied to the filter cake in the target production zone ina first stage and the mixture of the hydrochloric acid and the organicacid is applied to the filter cake in the target production zone in asecond stage, is operable to remove the filter cake in the targetproduction zone over an extended reaction time. Subsequently, thedelivery or application of the two-stage filter cake removal compositionis operable to control fluid loss from the wellbore into the targetproduction zone.

In accordance with an embodiment of the invention, the enzyme includesan enzyme that catalyzes the breakdown of starch into sugars to removethe polymeric material present on the Mn₃O₄ particles of the filter cakefor more effectively dissolving or removing the filter cake material inthe wellbore. According to an embodiment of the invention, the enzymeincludes an amylase enzyme present in an amount of between about 1% and20% by weight. The amylase enzyme may include, for example, an α-amylaseenzyme, a β-amylase enzyme, or a y-amylase enzyme. In a preferredembodiment, the enzyme of the two-stage filter cake removal compositionis an α-amylase enzyme present in an amount of about 10% by weight.

In accordance with another embodiment of the invention, the hydrochloricacid is present in an amount of between about 0.1% and 5% by weight, andmore preferably in an amount of between about 1 and 10% by weight.

In accordance with another embodiment of the invention, the organic acidincludes a carboxylic acid present in an amount of between about 0.1%and 5% by weight, and more preferably the organic acid is a carboxylicacid selected from the group consisting of formic acid, lactic acid, andmalonic acid. In one embodiment, the organic acid includes one of formicacid and lactic acid present in an amount of about 4% by weight.

In accordance with certain embodiments of the invention, the enzyme isapplied to the filter cake in the target production zone for apredetermined period of time based on one of a characteristic of thefilter cake, an enzyme type, concentration of the enzyme, and thethermal stability of the enzyme. For example, the enzyme may be appliedto the filter cake for a predetermined period of time of up to about 24hours, and more preferably for about 16-24 hours.

In accordance with certain embodiments of the invention, theconcentrations of the enzyme, the HCl, and the organic acid are based onone or more factors, including, but not limited to, the reservoirtemperature, formation mineralogy and composition, filter cakecharacteristics and composition, enzyme activity, and thermal stability.

EXAMPLES

The examples described below show certain exemplary embodiments of thefilter cake removal composition of the present invention, as describedherein. As shown in Table 1, water-based drilling fluids primarilyweighted with Mn₃O₄ and a small amount of CaCO₃ particles to control aleak-off rate were prepared to demonstrate the efficacy of the filtercake removal composition, in accordance with certain embodiments of theinvention, for dissolving or removing filter cake material in awellbore. Xanthan, starch, and polyanionic cellulose (PAC-R) polymerswere added to the mud to control fluid loss and rheological propertiesof the drilling fluid. Lime (Ca(OH)₂) and potassium hydroxide (KOH) wereadded to adjust the pH of the drilling fluid. Sodium sulfite (Na₂SO₃)was added to the drilling fluid as an oxygen scavenger.

TABLE 1 TABLE 1 - FORMULATION OF DRILL-IN FLUID Amount added Lab units(per ~320 cm³) Field unit (per bbl) Additive Function Quantity UnitQuantity Unit DI Water Base 287.7 cm³ 0.822 bbl Xanthan Viscosifier 1 G1 lb Starch Fluid loss control 6 G 6 lb agent PAC-R Viscosifier/fluidloss 0.75 G 0.75 lb KCl Density and shale 41 G 41 lb inhibition KOH pHcontrol 0.5 G 0.5 lb Ca(OH)₂ pH control 0.25 G 0.25 lb CaCO₃ Weightingmaterial 3.5 G 3.5 lb (Fine) CaCO₃ Weighting material 1.5 G 1.5 lb(Medium) Mn₃O₄ Weighting material 202 G 202 lb Na₂SO₃ Oxygen scavenger0.75 G 0.75 lb

Table 2 summarizes the main properties of the preparedmanganese-tetraoxide-based drilling fluid shown above in Table 1.

TABLE 2 TABLE 2 - PROPERTIES OF Mn₃O₄ DRILL-IN FLUID Property ConditionsUnit Value Density 80° F. and 14.7 psi lb/ft³ 95 Plastic 120° F. and14.7 psi cp 27 viscosity Yield point lb/100 ft² 38 API filtrate Staticat 75° F. and 100 psi cm³/30 min 6.5 Cake In 1/32 thickness HPHT Dynamicat 250° F. and 200 psi cm³/30 min 13.6-20.3 filtrate 3 μm, 400 mdceramic disc Cake In 0.24-0.29 thickness pH — — 10-11

According to various embodiments of the invention, HCl is effective fordissolving Mn₃O₄ particles and the Mn₃O₄-based filter cake. For example,as shown in FIG. 1, HCl in an amount of about 10% by weight may dissolveabout 78% by weight of a Mn₃O₄-based filter cake at about 250° F. andabout 250 psi after about 28 hours of soaking time (i.e., a filter cakethickness of about 0.171 in. prior to delivery or application of thefilter cake removal composition; a filter cake thickness of about 0.03in. after delivery or application of the filter cake removalcomposition). However, as discussed above, chlorine gas is detectedduring the reaction of between about 5 to 15% by weight HCl with Mn₃O₄particles. FIG. 2 shows a gas chromatography/mass spectrometry graphillustrating chlorine gas produced during the reaction of between about5 to 15% by weight HCl with Mn₃O₄ particles at about 284° F. As shown inFIG. 2, HCl should not be used at concentrations higher than 5% byweight in order to prevent the production of chlorine gas. At HClconcentrations lower than 5% by weight, however, the filter cake removalefficiency by weight is only about 50% by weight, and therefore is notmuch more efficient than conventional cleaning fluids.

Table 3 shows the solubility of Mn₃O₄ particles in HCl at 1%, 4%, and10% by weight HCl at about 190° F. Table 3 shows that a concentration ofHCl of about 4% by weight is effective, however, at dissolving a portionof the Mn₃O₄-solid particles in the filter cake (e.g., about 70% byweight) without producing chlorine gas, even though as shown in FIG. 2,this concentration of HCl does not allow for a filter cake removalefficiency that is much greater than conventional cleaning fluids.

TABLE 3 TABLE 3 - SOLUBILITY OF Mn₃O₄ PARTICLES IN HCl ACID SOLUTIONS,190° F. HCl Weight of retained Concentration, Dissolved solids afterreaction, Manganese wt % solids, wt % g Concentration, mg/l 1 56 1.76~7,500 4 70 1.2 ~8,500 10 100 0 ~18,000

Embodiments of the invention further demonstrate that organic acids maybe used to dissolve or remove Mn₃O₄ particles from the filter cake. Asshown in Table 4, at about 4% by weight acid concentration, organicacids, such as lactic and formic acids, as non-limiting examples, maydissolve Mn₃O₄ particles up to 76% by weight solubility at about 190° F.Malonic acid may be used to dissolve Mn₃O₄ particles in the filter cake,but it may not be as effective as formic and lactic acids (i.e., onlyabout 50% by weight of the Mn₃O₄-solid particles in the filter cake aredissolved).

TABLE 4 TABLE 4 - SOLUBILITY OF Mn₃O₄ PARTICLES (4 g) IN ORGANIC ACIDSSOLUTIONS (200 g), 190° F. Acid type Dissolved Manganese (concentration,solids, Concentration, % of wt %) wt % mg/l Consumed acid 4 wt % lacticacid + 99 14,600 at 10 min 66.5 1 wt % HCl Formic (4) 76.4 11,000 at 70min 52.6 Lactic acid (4) 70.2  8,900 at 35 min 75.9 Maionic (2) 53.73 6,600 at 25 min 60

In a preferred embodiment of the invention, a mixture of HCl, forexample, in an amount of about 0.1% and about 5% by weight, and theorganic acid, for example, in an amount of about 0.1% and about 10% byweight, is delivered or applied to the filter cake in a targetproduction zone of a wellbore. As shown in Table 4, for example, amixture of 1% by weight HCl and 4% by weight lactic acid may dissolveabout 99% by weight of the Mn₃O₄-solid particles in the filter cake. Aspreviously discussed, this mixture of HCl and lactic acid may beoperable to dissolve about 85% by weight of the filter cake.

Certain embodiments of the invention provide a two-stage filter cakeremoval composition for controlled removal of a filter cake present in atarget production zone. As previously discussed above, Mn₃O₄ particlesin the filter cake may be covered with polymeric material (e.g.,starch), which significantly reduces the solubility of the filter cakein organic acids. In order to remove the polymeric material, embodimentsof the invention provide a two-stage filter cake removal compositionwhich includes the application of an enzyme in a first stage and theapplication of a mixture of HCl and an organic acid in a second stage.

Experimentation demonstrates that an amylase enzyme in an amount ofabout 10% by weight and an in-situ lactic acid in an amount of about 10%by weight may dissolve about 84% by weight of the filter cake at about244° F. after soaking the filter cake in the organic acid for about 24hours (see FIG. 4). The enzyme soaking stage time is about 4 days.Dissolving the filter cake created by Mn₃O₄ water-based drilling fluidsusing a two-stage treatment may increase the time of the treatment(e.g., 1-4 days for the enzyme stage alone) depending on variousfactors, for example, filter cake polymeric composition, enzyme type,concentration and thermal stability, as non-limiting examples. AddingHCl to the second stage of the treatment reduces the required enzyme andorganic acid concentrations and the soaking time to about 24 hours foreach stage.

For example, the two-stage filter cake removal composition, inaccordance with certain embodiments of the invention, provides completesolubility of the Mn₃O₄ particles (e.g., about 99% by weight) in thefilter cake, when a mixture of low and safe concentrations of HCl, forexample, in an amount of about 1% by weight, and an organic acid, forexample, of about 4% by weight of lactic acid, is applied to a filtercake after the filter cake is treated with an initial stage of anenzyme, for example, an amylase enzyme in an amount of about 10% byweight, which dissolves about 85% by weight of the Mn₃O₄-based filtercake after about 18 hours soaking time at about 250° F.

FIG. 5 shows a graph illustrating that a mixture of HCl and lactic acidmay be more effective in the dissolution of manganese ions from theMn₃O₄-based filter cake than with a treatment of only lactic acid oronly HCl, in accordance with an embodiment of the invention. FIG. 6shows a graph illustrating the solubility of the Mn₃O₄-based filter cakeand acid concentration as a function of time using the mixture of about1% by weight HCl and about 4% by weight lactic acid at about 250° F. andabout 250 psi. FIG. 7 shows a filter cake thickness of about 0.174 in.prior to delivery or application of the single-stage filter cake removalcomposition, in accordance with an embodiment of the invention, and afilter cake thickness of about 0.026 in and about 0.024 in. aftersoaking the Mn₃O₄-based filter cake in the two-stage filter cake removalcomposition for about 18 hours and about 22 hours, respectively. Noenzyme was used in the filter cake removal composition represented byFIGS. 6 and 7.

Embodiments of the present invention may suitably comprise, consist orconsist essentially of the elements disclosed and may be practiced inthe absence of an element not disclosed. For example, it can berecognized by those skilled in the art that certain steps can becombined into a single step.

Unless defined otherwise, all technical and scientific terms used havethe same meaning as commonly understood by one of ordinary skill in theart to which this invention belongs.

The singular forms “a,” “an,” and “the” include plural referents, unlessthe context clearly dictates otherwise.

As used herein and in the appended claims, the words “comprise,” “has,”and “include” and all grammatical variations thereof are each intendedto have an open, non-limiting meaning that does not exclude additionalelements or steps.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value and/or to the other particular value, along withall combinations within said range.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions, and alterations canbe made hereupon without departing from the principle and scope of theinvention. Accordingly, the scope of the present invention should bedetermined by the following claims and their appropriate legalequivalents.

What is claimed is:
 1. A filter cake removal composition for use in awellbore for controlled removal of a filter cake present in a targetproduction zone, the filter cake removal composition comprising:hydrochloric acid; and an organic acid, wherein the filter cake removalcomposition, when a mixture of the hydrochloric acid and the organicacid is applied to the filter cake in the target production zone, isoperable to dissolve the filter cake in the target production zone overan extended reaction time, and wherein the mixture comprises thehydrochloric acid present in an amount of between about 0.1% and 5% byweight and the organic acid present in an amount of between about 0.1%and 10% by weight.
 2. The filter cake removal composition of claim 1,wherein the hydrochloric acid is present in an amount of about 1% byweight.
 3. The filter cake removal composition of claim 1, wherein theorganic acid is selected from the group consisting of formic acid,lactic acid, and malonic acid.
 4. The filter cake removal composition ofclaim 1, wherein the organic acid comprises lactic acid present in anamount of about 4% by weight.
 5. The filter cake removal composition ofclaim 1, wherein the filter cake removal composition comprises themixture of the hydrochloric acid present in an amount of about 1% byweight and the organic acid present in an amount of about 4% by weight.6. The filter cake removal composition of claim 1, wherein the mixtureof the hydrochloric acid and the organic acid is applied to the filtercake in the target production zone in a single treatment for about 18hours at about 250° F.
 7. A method for the controlled removal of afilter cake from a target production zone of a wellbore using a filtercake removal composition, the method comprising: delivering the filtercake removal composition to the target production zone, wherein thedelivering comprises applying the filter cake removal composition to thefilter cake for an extended reaction time during which extended time thefilter cake removal composition acts to dissolve the filter cake andafter which extended time the filter cake removal composition acts tocontrol fluid loss from the wellbore into the target production zone,wherein the delivering comprises applying a mixture of hydrochloric acidand an organic acid to the filter cake in the target production zone,and wherein the mixture comprises the hydrochloric acid present in anamount of between about 0.1% and 5% by weight and the organic acidpresent in an amount of between about 0.1% and 10% by weight.
 8. Themethod of claim 7, wherein the delivering further comprises applying themixture to the filter cake in the target production zone, wherein themixture comprises the hydrochloric acid present in an amount of about 1%by weight.
 9. The method of claim 7, wherein the delivering furthercomprises applying the mixture to the filter cake in the targetproduction zone, wherein the mixture comprises the organic acid beingselected from the group consisting of formic acid, lactic acid, andmalonic acid.
 10. The method of claim 7, wherein the delivering furthercomprises applying the mixture to the filter cake in the targetproduction zone, wherein the mixture comprises the organic acid presentin an amount of about 4% by weight.
 11. The method of claim 7, whereinthe delivering further comprises applying the mixture of thehydrochloric acid present in an amount of about 1% by weight and theorganic acid present in an amount of about 4% by weight.
 12. The methodof claim 7, wherein the delivering further comprises applying themixture of the hydrochloric acid and the organic acid to the filter cakein the target production zone in a single treatment for about 18 hoursat about 250° F.